Crohn's disease (CD) is a chronic and debilitating inflammatory bowel disease with unknown etiology; however, it is clear that there are both genetic components as well as environmental factors that are required for its development. Tremendous advances have been made recently in understanding the genetics of CD. Although several chromosomal loci have been associated with IBD susceptibility, until last year, only one gene had been conclusively identified as a CD susceptibility gene. This gene is NOD2 (nucleotide-binding, oligomerization domain 2), which encodes an intracellular sensor of bacteria in the innate immune system. Recent genome-wide association scans have identified multiple, novel CD susceptibility genes. Many of these new CD susceptibility genes also modulate immune responses, suggesting that dysregulation of an immune pathway rather than a single gene may be important in CD pathogenesis. One of these processes is autophagy. Autophagy (literally self-eating) is best known as a cell survival mechanism in response to starvation, where organelles are broken down and recycled to provide nutrients. This mechanism also plays a crucial role in the clearance of intracellular bacteria by the innate immune system. Bacteria and bacterial components induce autophagy, linking innate immune microbial sensors to autophagy. One of these microbial sensors is Nod2, which detects a specific component of the bacterial cell wall called muramyl dipeptide (MDP). Our preliminary data demonstrates that stimulation of cells with MDP not only activates autophagy, but suggests autophagy may also be involved in activation of Nod2 by MDP. These results provide a link between two separate classes of molecules mutated in CD and lead us to propose the following central hypothesis: CD- associated mutations in the autophagy genes, ATG16L1 (autophagy related 16-like protein 1) and IRGM (immunity-related GTPase family, M), and NOD2 impair the same critical innate immune pathway and impairment of this pathway contributes to CD pathogenesis. We hypothesize autophagy and Nod2 function together in two non-exclusive ways: (1) MDP stimulation of Nod2 activates autophagy as an essential anti-microbial response, and (2) autophagy is involved in MDP internalization and Nod2 activation. This hypothesis will be tested by the following three specific aims using both primary cells and cell lines: 1) Determine the role of Nod2 in activation of autophagy - is autophagy a component of Nod2 stimulated anti- microbial responses?, 2) Characterize the mechanism of MDP internalization - is it autophagy?, 3) Analyze alterations in autophagy in phagocytic cells of CD patients. The results of our proposed studies may benefit individuals with CD by identifying a crucial processes altered in CD, leading to the design and use of more specific CD therapies targeting this pathway.